Field of the Invention
This invention relates to astrophotography, and more particularly to tracking devices used to enable long exposures when performing astrophotography.
Description of the Related Art
Astronomers have been photographing the night sky for many decades. In the early years of astrophotography film emulsions were not very sensitive, and telescope mountings were designed to allow exposures hours in length while the mount accurately followed the stars. The rotation of the earth is approximately 15 arc-seconds of angle per second of time, and the accuracy of tracking must be on the order of 1 arc-second of angle to not blur the resulting image. Many types of telescope mountings were developed to solve this problem, such as the German Equatorial, or an Equatorial Fork design, for example. Many books and publications describe the various permutations of these designs. A common characteristic of these designs is that they have one rotation axis accurately aligned on the North Celestial Pole (near the star Polaris), and the other perpendicular to that axis. The simplicity of such a design is that the image on the photographic plate does not rotate about its center point over time. However, such designs usually have off-axis mounting for the telescope and substantial weight displaced from the centerline of the mounting base, requiring heavy counterweights. Modern telescopes at large observatories have evolved to a design where the telescope axes are in what is called an Alt-Az configuration. One axis is vertical and allows motion in azimuth, and one is horizontal, providing motion in elevation. However, with such a design the image rotates, so an image rotator is used at the focus position of the telescope to compensate for that motion. The image rotation is calculated based on the knowledge of the telescope geometry and the object's position in the sky, a fairly complex real-time trigonometric calculation that is the reason why this approach was never used before the dawn of the computer age.
In recent years, cameras have evolved that are vastly more sensitive than was the case in the early days of film, and modern lenses are also quite fast photographically, with an F/1.4 or F/2 lens being common. As a result, an amateur astrophotographer with a digital camera employing a CCD or CMOS detector can capture excellent photographs of the night sky with exposures of less than 30 seconds. The sensitivity of the camera is approaching the theoretical limits. However, the astrophotographer soon discovers that when he or she mounts his camera on a fixed tripod, that the rotation of the sky blurs the images fairly quickly. For example, with a lens of 50 mm focal length, exposures longer than 1.5 seconds show streaking. The amateur can improve this situation by using shorter focal length, wide angle lenses, but still with any lens other than a fisheye lens exposures are limited to around 10 seconds. This solution is adequate for very wide angle astrophotography, but disappointing for longer lens used to photograph smaller objects.
Camera manufacturers have attempted to solve this problem by using optical elements in the camera that can translate the image falling on the sensor, or even translate the sensor itself. An approach involving translating the sensor is disclosed by Ohta in U.S. Pat. No. 8,717,441. A few other camera patents utilize a similar approach: two sequential images are taken of a star field to determine the direction and rate of drift of the image on the sensor, and then an exposure is started and “guiding” employed to minimize the blur due to the earth's rotation. This is not true guiding since the process is open loop; that is, no images are captured while the exposure is in progress, resulting in small errors that eventually accumulate and corrupt the image. While this allows the user to capture an exposure 10 times longer than otherwise, it still has an exposure limit of 10 to 30 seconds.
To solve this problem, the astrophotographer can either mount the camera on a telescope mount to follow the sky, or use a smaller camera tracker unit that has a clock drive to compensate for the earth's motion. In both cases an axis must be aligned on the north celestial pole to good accuracy. Usually this is done by including a small telescope that views Polaris as an aid in getting aligned close to the pole quickly. However, the star Polaris is 0.7 degrees away from the pole, but the alignment on the pole must be made to an accuracy of 0.1 degree to enable a 5 minute exposure with a 300 mm focal length lens. A reticle eyepiece in the small scope can be used to compensate for off-pole offset of Polaris. This technique works more poorly in the Southern Hemisphere, where Polaris is not visible and there is no comparable bright star. Some telescope manufacturers have developed star field recognition accessories to help an amateur align a telescope without a clear view of Polaris. These work well, but the amateur then confronts another problem. Most telescope drives, in long exposures, show periodic error in their main worm gear driving the polar axis compensating for the sky, or have some deflection in their structures that result in small errors creeping into the guiding and blurring the images. To compensate for this, a CCD is usually used to guide on a star nearby to the field being photographed, reducing this error to zero. So, quality astrophotography is achieved, but at the expense of a heavy, complex system. Now the problem for the amateur becomes transporting this bulky system to a dark sky site, perhaps a long plane flight away in the Southern Hemisphere, or up a mountain trail to a hilltop. A system with total weight between 25 and 50 pounds would be common, all to guide a one pound camera, and that ignores battery weight for remote operation.